Why are my object's shadows so janky?
The reason this mesh has strange shading is because of the topology.

Polygons which are made of more than 4 vertices are called ngons. These ngons are pretty extreme in your mesh as you can see. While ngons have their place (on totally planar faces) if an ngon face has even the slightest twist, then it will cause horrible shading.
This is why in 3D modelling (especially for beginners) people avoid ngons and aim to keep faces consisting of no more than 4 vertices, which are called quads.
Another way to make these kinds of stylized rocks is to add a cube. Subdivide it with constant detail and then enter sculpt mode.
With the scrape brush, you can scrape away at the faces of the heavily subdivided cube to get nice flat edges.
Once you're happy with it, you can leave sculpt mode and then add a decimate modifier and decimate the rock to reduce the polygon as much as possible (lower the value in the slider to somewhere near 0.01).
The decimate modifier will retain the detail best while keeping the shading looking the best. Once you find a happy medium between detail and polygon count, you can apply the decimate modifier and move on to UV unwrapping and later texturing.
This is how I make stylized rocks.
Hope that helps! :)
I just added a rock I did just now.
https://www.dropbox.com/s/rcbo7lwoyoetbln/RockSculpting_Demo.blend?dl=0
I just rushed this so with a bit more time and effort put into it, you could get something looking nicer but hopefully it'll give you an idea of how the topology should look.